Melissa McCarthy’s performance as White House press secretary Sean Spicer on last weekend’s “Saturday Night Live” was the show’s best satire in ages. Sharp as Dexter Morgan’s scalpel, the chaotic press-room scene was intelligently written and acted with an archer’s focus. It’s since been watched on YouTube more than 19 million times.

It also raised a Big Red flag.

NBC viewers are starting to catch on to “SNL’s” No. 1 problem: None of the show’s most memorable moments belongs to regular cast members anymore.

Throughout the 2016 election cycle and in the early days of the Trump administration, the winningest sketches have been anchored by celebrities who are not part of the Not Ready For Prime Time Players: Alec Baldwin, Larry David and now McCarthy. Besides Kate McKinnon’s brilliant Hillary Clinton and Kellyanne Conway, the ensemble’s character repertory is slim as Shady.

While it’s a hoot to watch Baldwin moan and mug as Donald Trump or David rattle on like Bernie Sanders, the crippling reliance on celebrity cameos magnifies why “SNL” is not consistently funny these days.

Melissa McCarthy as Sean SpicerWill Heath/NBC

What’s the diagnosis? Well, its main cast consists largely of stand-up comedians and self-absorbed improvisers with middling to nonexistent acting chops. Trudging through the viscous swamp of impersonations, they give it the old college try, but performers like Michael Che, Leslie Jones and Pete Davidson can’t quite shed a comic’s cutesy self-awareness. That style can be pleasant for a minute on YouTube, but definitely not for eight in front of a live audience.

As a result, few of the players are gaining the affection of viewers. With all the cast comings and goings — 13 new featured players have been added since 2013, five of whom lasted only a single season — Studio 8H is practically a temp office.

Their issue has nothing to do with physical likeness. McCarthy, obviously, is a woman. Her victim, Spicer, is a card-carrying, gum-eating dude. But McCarthy, one of our most gifted comedy actresses, harnessed his self-defensive essence in spite of their visual differences. The greatest “SNL” actors can.

During his many years on the show, Darrell Hammond didn’t really look like anybody, but he could easily transform into Al Gore, John McCain, Dick Cheney or Bill Clinton. That was also true of Dana Carvey and his excellent George H.W. Bush impersonation. The same skills it takes to inhabit a real-life person are required to play any character, really.

You gotta find out what makes him tick, not just what his tics are.

Now there is even a movement to get Rosie O’Donnell to appear on the show as White House chief strategist Steve Bannon. Apparently viewers think the Not Ready For Prime Time players aren’t ready for late night, either